Transformer Winding Temperature

[sambapati]

Transformer winding temperature indicator (WTI) zero temperature trip. Recently, our transformer WTI made by MSRT Terman tripped swhen the capillary tube leaked and temperature reading dropped to zero. We did not know this feature of the WTI thus we opened it and found that the design would really trip at zero temperature. We proposed that we place some barrier in the meter to prevent the temperature from dropping to zero in case the tube again leaks. Is this OK?

 

[Compositepro]

If you do that, then when the tube breaks you will not get a shutdown, but you will no longer have any over-temperature protection, and you will not know it.

 

[sambapati]

Thank you very much for the reply. May I know if this zero temp tripping is standard among WTI's? We have Thyoda made in Japan and AKM Qualitrol as our other brands of WTI. What standards do I have to use to refer this issue?

 

[edison123]

They trip to indicate that the WTI is broken?

Muthu

http://www.edison.co.in

 

[prc]

I do not know this particular make of WTI. How this can be accepted? As per IEC 60076-1 on transformers, transformers shall be suitable for service up to -25C. So WTI &OTI can go below 0C. Even if WTI is inoperative, transformer need not trip. OTI is more realistic for temperature alarm and trip than OTI.

 

[7anoter4]

As prc said IEC 60076-1 states the minimum ambient air at -25 oC.[ 1.2.1 Normal service conditions b) Temperature of ambient air and cooling medium].
If we shall say for full load winding temperature rise will be 90oC then at no-load [1/6 losses of full load] the temperature rise will be 90/6=15 oC. So at -25oC ambient a transformer at no-load winding temperature will be -10oC.
If no other mean will warn us about tube breaks-as Compositepro said-and then the WTI could not signalize an overheating- a minimum -10oC setting on WTI relay will solve the issue, in my opinion.

 

[bacon4life]

What size transformer is this? Tripping the transformer for a temperature gauge failure seems pretty extreme.

Using a setting of -10C would interfere with energizing the transformer in colder weather. You may want to consider the case of cold load pickup after an outage.

 

[sambapati]

Thank you very much for the responses. Our utility is in a tropical country thus our transformer WTI and OTI gauges are scaled from 0 to 150 deg. Our initial action was to revise our fan control scheme so that the OTI can also start run the cooling fans instead of the WTI only. We have also included in our maintenance inspection the leak check test of the capillary tubes near the well on top of the transformer where our previous leak occurred. Incidentally, our transformer rating is 83 MVA, 115kV-34.5kV-13.8kVD and 65 deg temperature rise.